Puigjaner L. (ed.) Syngas from Waste

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electrolytes, Henry’s law and complex activity coefﬁcient formulations which are

then replaced by simpler split fractions for separation processes. In the case of

reactions it also relies on chemical equilibrium or on reaction degrees of advance

based on previously modelled exhaustive models. Due to a longer units train, and

even though the LHV of H

is higher than that of syngas, the ﬁnal energy content

of the stream that goes to the CC has a lower caloriﬁc value. After the PSA process

there exists a ﬂue stream with high energy potential, thus suitable to be introduced

into the gas turbine.

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Main Puriﬁcation Operations 119

Emerging Technologies on Syngas

Puriﬁcation: Process Intensiﬁcation

Ramón Álvarez-Rodríguez and Carmen Clemente-Jul

Abstract Syngas normally contains a series of contaminating gases, depending on

the raw materials used, the most abundant one usually being H

S, accompanied by

COS and, also, HCl, HF, etc. Normally, puriﬁcation should be performed before its

combustion in the gas turbine (in the case of combined cycle plants) and the classic

procedure, as performed at present in some installations, uses the wet process,

which demands a reduction in the temperature of the gas to be puriﬁed and,

therefore, gives rise to a series of thermodynamic losses. The trend is to research

high-temperature puriﬁcation processes that avoid or reduce this loss in perfor-

mance. In particular, there are two research lines for sulphur compounds: (i) The

use of low-value metal adsorbents that may be discarded once they have been

stabilised and without contaminating properties, such as calcium compounds that

may produce CaO that captures the hydrogen sulphide and (ii) the ‘important

value’ adsorbents that therefore require the ability to be regenerated and reused,

and whose base are metals with high afﬁnity with sulphur, such as Zn, Fe and Cu

but whose cost is much higher than the previously mentioned calcium compounds.

Notation

IGCC Intergrated Gasifuration Combined Cycle

MDEA Methyldiethanolamine

R. Álvarez-Rodríguez (&)  C. Clemente-Jul

ETSI Minas, Alenza 4, Madrid, 28003, Spain

e-mail: ramo.alvarezr@upm.es

C. Clemente-Jul

e-mail: Carmen.clemente@upm.es

L. Puigjaner (ed.), Syngas from Waste, Green Energy and Technology,

DOI: 10.1007/978-0-85729-540-8_6,  Springer-Verlag London Limited 2011

121

1 Adsorption of Sulphur Compounds Over Solid Adsorbents

1.1 General Considerations

In reactions with the intervention of solid sorbents, it is very important for possible

practical applications to consider the inﬂuence of the different state of aggregation

of the reactants and the reaction products. In the case of syngas, the reactant to be

eliminated is in a gaseous state and the eliminating substance is in solid state. The

reaction products will normally be solid or gaseous (water is in vapour form at

high temperatures).

For the sorbent, in the form of solid particles, it is sought for it to react as fast and

completely as possible. Initially, the action of gases on particles occurs without any

problem on their surfaces but, afterwards, the reaction must progress towards the

grain core. This will be according to its porosity and the size distribution of its pores,

it is also important to consider its size when considering the reaction at the grain core

because, if it is very large, the core will be far away from the surface, hindering the

passage of reactant gases and those produced. This justiﬁes the sorbent preparation

methods seeking an improvement in these porosity characteristics.

For the process to occur normally, the solid product formed in the reaction must

have a molar volume such that the pores are not obstructed, which would create a

gas-tight layer preventing the reaction from progressing towards the core.

On the other hand, the gases produced on exiting the grain to become part of the

general gas current must pass through the layer that has already reacted and, at this

stage, secondary reactions may occur in some cases because there is good contact

between the new gas produced (if it is the case) and the new solid in its pores. This

is outlined in Fig. 1.

In addition, the grains of the sorbent can be found more or less packed

depending on the type of reactor in which the reaction takes place. In this way,

there will be strong packing in a ﬁxed or moving bed, whereas this will not be the

same in a ﬂuidised bed. The grain size distribution of the sorbent and its bed height

will have an inﬂuence in the case of packing, because the porosity of the column

will be low with very low grain size distributions and preferential gas passage

paths may be created, leaving areas with bad gas supply that will partly cancel the

advantages of small grain sizes having a more complete reaction, while also

increasing a drop in the pressure necessary for a speciﬁc gas ﬂow.

There may also be an inﬂuence between the different grains of the sorbent,

being greater the more packed they are, as it can be seen in Fig. 2. If a product

from the reaction of the ﬁrst grains is a gas, it ﬂows in the direction of the gas ﬂow

and will go on into contact with the next ones and, if there is any possibility, there

may be secondary reactions.

Another important issue, particularly in the cases in which the sorbent is to be

regenerated and used during several cycles, is to maintain its mechanical resistance,

together with its porosity. Because of the alternative changes in composition of the

solid, advancing cracks or splits could appear, mechanically degrading the sorbent and

122 R. Álvarez-Rodríguez and C. Clemente-Jul

creating ﬁnes that hinder the passage of gases, dragged along, and are also, continu-

ously subjected to high temperatures favouring sorbent sintering, leading to a gradual

reduction in its porosity and, as a result, in its efﬁciency. This is the reason for using

sorbents that can be regenerated in these cases, using product mixtures which, with an

adequate preparation process, on one hand improves the porosity and on the otherhand

contributes towards an improvement in mechanical resistance and resistance to sin-

tering, which explains the large variety of proposals in existence regarding this issue.

1.2 Adsorption of Sulphur Compounds Over Cheap Solid

Adsorbents

In the production of syngas, there is an entire range of contaminants that pollute

gas among which sulphur compounds should be stated as the main ones, as this

element is present both in coal and in petroleum coke, heavy fuel and in the

Possibility of secondary reactions on other grains

Gas in

Gas produced

Fig. 2 Possibility of

secondary reactions over the

solid-transformed outer shell

of other grains

Initial case

With a transformed shell

Possibility of secondary

reactions in solid

Gas produced.

No secondary

reactions in solid

Gas in

Fig. 1 Possibility of

secondary reactions in a

solid-transformed outer shell

Emerging Technologies on Syngas Puriﬁcation: Process Intensiﬁcation 123

majority of residual organic materials that can be used as raw material for gasi-

ﬁcation, including animal organic waste. The content of these contaminants

basically depends on the composition of the mixture to be gasiﬁed.

In reducing conditions, sulphur compounds are mainly H

S but also, to a lesser

extent, COS. Both produce SO

in the case of combustion and, therefore, it is

convenient to eliminate this before the combustion process to reduce problems

regarding corrosion in pipes, equipment and particularly in the turbines as the

tendency is to use gas turbines in combined cycles to obtain greater energy efﬁ-

ciency. For other applications, such as obtaining H

for fuel-cells, the elimination

of sulphur must reach very low values, normally under 1–10 ppm.

The elimination of these contaminants is currently carried out in the combined

cycle installation (IGCC) of Elcogas (Puertollano, Spain) [1] with water cleaning

in a Venturi type set of equipment and later H

S through extraction with

Methyldiethanolamine (MDEA), but requiring a reduction in the temperature of

the gas, which leads to inherent losses in performance. In this case, COS is

eliminated in a previous hydrolysis stage, transforming it into H

S for it to be

retained by the MDEA.

Conventional wet process technologies are proven ones that are almost fully

available although, as stated earlier, they are thermally inefﬁcient.

The use of cheap adsorbents is essentially based on the use of calcium

oxide found in two mineral substances widely present in nature, such as

dolomite and calcite minerals. These compounds, such as calcite or CaO, have

been used to eliminate the SO

caused by burning the sulphur or its com-

pounds contained in the fuels of thermal power plants. There are many

researches trying to increase the reactivity of these compounds by means of

mixing with other products and special preparation methods to increase their

porosity and reactivity [2].

On the other hand, syngas has a variable composition, to a large degree con-

taining CO, CO

and to a lesser extent H

S, COS, together with some other

compounds, such as HCl and HF.

As a whole, there are a series of reactions and their corresponding balances

controlling the composition of the gas, the main reactions in which the solids

participate are the following:

CaCO

$ CaO þ CO

¼þ178:3 kJ mol

1

ð1Þ

CaMgðCO

$ CaO þ MgO þ 2CO

¼þ302:7 ð2Þ

CaO þ H

S $ CaS þ H

O DH

¼59:2 kJ mol

1

ð3Þ

CaCO

þ H

S $ CaS þ CO

¼þ119:1 kJ mol

1

ð4Þ

COS þ CaO $ CaS þ CO

¼93 kJ mol

1

ð5Þ

The main reactions between the main gaseous compounds and those implying

the reaction with sulphur compounds are the following:

124 R. Álvarez-Rodríguez and C. Clemente-Jul

þ H

$ CO þ H

O DH

¼þ41:1 kJ mol

1

ð6Þ

þ H

S $ COS þ H

OgðÞ DH

¼þ33:6 kJ mol

1

ð7Þ

þ 2H

S $ CS

þ 2H

OgðÞ DH

¼þ68 kJ mol

1

ð8Þ

CO þ H

S $ COS þ H

¼7:4 kJ mol

1

ð9Þ

Reactions (1) and (2) imply that the minerals we consider may lose their CO

high temperatures to provide the corresponding oxides. The temperature at which

this occurs is normally high and depends on the speciﬁc mineral and also on the

partial pressure of CO

, these reactions being reversible. This decomposition

temperature is lower in dolomites than in calcites, with the possibility of reaching a

state of semi-calcining in the dolomite.

Once these minerals have been calcined, and as the reaction is reversible, CaO

may be recarbonated in the presence of CO

at low temperatures, such as 400 or

650C as syngas contains a certain proportion of CO

. Certain processes for CO

sequestration from ﬂue gases in thermal power stations are based on this

reversibility.

Reactions (3) and (4) imply that the hydrogen sulphide can react both with the

calcium oxide and with the carbonate to turn into calcium sulphide and, therefore,

retains this gas. Water or water and carbon dioxide are produced by these reac-

tions, which go on to form part of the gaseous mass, modifying their proportions in

terms of the balance constants controlling the composition of such gaseous state.

MgO does not react with H

S under the sulphidisation conditions.

In ﬂuidised bed gasiﬁers (normally 800–1,050C), the standard practice to

retain H

S is to enter these minerals, mainly calcite, in the reactor and under the

corresponding temperature conditions and depending on these, therefore being in

non-calcining or calcining conditions, the reactions (3) and (4) take place.

Normally, it is necessary to add an excess amount of calcite in order to reach

good H

S elimination conditions and to get close to the balance values, which

translates into the appearance of solid waste residue with a notable proportion of

calcite [3, 4]. Yrjas [5] studying the capture of H

S by calcite (limestone) and

dolomite in a pressurised thermogravimetric apparatus found that 0.125–0.180 mm

particles at 950C and 2 MPa in non-calcined calcite and half-calcined dolomite,

dolomite was signiﬁcantly more efﬁcient than calcite. With calcined dolomite and

calcite, capture was fast and highly efﬁcient.

Pinto et al. [6] gasifying mixtures of Puertollano (Spain) coal (1.2% S), pet-

coke (6% S), pine waste (0.2% S) and polyethylene waste (0% S), using a pilot

gasiﬁcation plant with a bubbling ﬂuidised bed at 850C, found that the proportion

of sulphur going into the gaseous state strongly depends on the sulphur contents of

the mixture. Mixtures of sorbents or catalysts with sand from the bed (calcined

dolomite, dolomite enriched with Ni [7], one called Ni–Mg [8], natural and cal-

cined olivine and other commercial ones) are used to study the sulphur retained

and the reduction in tars and hydrocarbons. The greatest reduction in sulphur in the

Emerging Technologies on Syngas Puriﬁcation: Process Intensiﬁcation 125

gaseous state is with dolomite (90%), being lower with Ni-dolomite, very low with

Ni–Mg and calcined olivine and not at all with natural olivine. At 900C, the

proportion of sulphur going into the gaseous state increases and dolomite is the one

retaining the most. These adsorbent compounds have also been used for the pro-

duction of hydrogen from coal [9].

In entrained-ﬂow type gasiﬁers, the temperatures reached are much higher and,

normally, calcite additions to the mixture to be gasiﬁed are basically used to

control the ﬂuidity conditions of the slag extracted in molten state. In these gas-

iﬁers, the greater part of sulphur from feeding goes into gaseous state and only a

small part is eliminated in the condensed state. In the IGCC of Puertollano, it has

been determined that the passage of sulphur to the gaseous state is of around 95%

of the contents during feeding [10] or 68–96% depending on the conditions (Font

et al. 2010) and that the greatest part of that is retained in the slag, essentially as

troilite (FeS) and pyrrhotite (Fe

1-x

S) and, to a lesser extent, in the ﬂy ash as

pyrrhotite and also galena and sphalerite, if there are Pb and Zn in the mixture to

be gasiﬁed, which usually occurs with coal and pet-coke mixtures, and there seems

to be no correlation between the proportion of sulphur going into the gaseous state

and calcite addition (Font et al. 2010).

There are many studies on the adsorption of H

S in beds containing CaO, some

of which have been carried out using very small amounts of oxides (from 1 mg to

1 g) to obtain the initial inﬂuence of the main parameters or to obtain mathe-

matical models [11–13]. In other cases, greater amounts of adsorbent were used

but mixed with silica to obtain a better distribution of the gas ﬂow within the mass

of the adsorber and to obtain a better proﬁle conversion, also with the purpose of

obtaining results for the preparation of mathematical models [14].

Fig. 3 Breakthrough curves obtained at calcining conditions, 900C, (open circles) dolomite,

(open squares) ‘sástago’ limestone and (open triangles) ‘omyacarb’ limestone, and non-calcining

conditions, 850C, (ﬁlled circles) dolomite, (ﬁlled squares) sástago limestone and (ﬁlled

triangles) omyacarb limestone. Grain size 0.8–1.0 mm, 1.0 MPa, 18.5 cm bed length [14]

126 R. Álvarez-Rodríguez and C. Clemente-Jul

Greater amounts, from 100 to 150 g, were used other times without mixing with

inerts so as to obtain a better approximation to their real use [15].

Adánez et al. [14] have already shown, Fig. 3, that in experiments at 1 MPa and

0.5 vol% of H

S, 18.5 cm of bed length and 37 cm s

-1

gas velocity and using

dolomite and calcite from Aragón (Spain) at 900C, there is a greater conversion

of dolomite (almost a full conversion) than that of calcite (85%), making the

breakthrough curve of dolomite appear to the right to that of calcite. At 850C

(non-calcining conditions), the behaviour of dolomite can be stated to be similar to

that of calcite at 1,173K but that the behaviour of calcites at this temperature of

1,123K is much worse and only very low conversions are achieved.

In general, the better behaviour of dolomite than that of calcite can be

explained, apart from the lower calcining temperature, by the fact that the mixture

remains after emitting the CO

is one of MgO and CaO and, as MgO does not react

with H

S, it leaves a greater porosity available for the access of H

S towards the

core of the grains. Figure 4 shows the diffractogram of a calcined dolomite from

Granada (Spain) in which it can be seen that the ﬁnal composition is of CaO

(Lime) and MgO (periclase).

The use of dolomite instead of calcite, therefore, seems advantageous and, with

regard to the adsorption of H

S in a ﬁxed bed, there are several considerations that

should be made [15]: An important issue is the grain size distribution of the

dolomite because ﬁne grains (e.g., less than 1 mm) cause a non-uniform gas ﬂow

distribution in the bed, therefore, creating preferential paths for gas passage and

giving rise to lower efﬁciency.

With regard to the H

S adsorption, the breakthrough curve presents an initial

section corresponding to a very low concentration of H

S in the output gas that

later increases progressively until the adsorbent becomes saturated and the input

and output become equal. This can be seen in Fig. 3. Considering a possible

industrial use, this initial section of the curve will condition the minimum H

contents that may be achieved in the output gas, and once a certain level is set, the

adsorber, still with capacity to adsorb H

S, cannot continue to be used as an output

stage for the gas to be puriﬁed and, by means of a valve system, the output gas will

Fig. 4 Calcined granada

dolomite, L lime, P periclase

Emerging Technologies on Syngas Puriﬁcation: Process Intensiﬁcation 127

Puigjaner L. (ed.) Syngas from Waste - Emerging Technologies

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